Increasing concern over the daily emission of both toxic and greenhouse gases has prompted the need for more sensitive and selective gas sensors. Legislation restricting the current and future emission of these gases from sources such as internal combustion engines indirectly requires the improvement of sensors to detect gases such as carbon dioxide, carbon monoxide, and nitrous oxide. Modern sensors rely on the interactions between sensor and analyte to produce a detectable change in either the electrical, optical, mechanical, electrochemical, or thermal properties of the sensor. This interaction can take the form of physisorption, chemisorption, or catalysis.
A sensor that relies on physisorption has the advantage of being easily refreshed. However, physisorption involves weak attractive forces which present difficulties in establishing analyte detection. Chemisorption sensors rely on the chemical bonding of the analyte molecules to the probe surface to produce a detectable change in the electrical conductivity mass, capacitance, or optical properties of that surface. Difficulties arise when the sensor is in continual use as it is necessary to expose a fresh probe surface to the incoming analyte due to the chemical bonding of the analyte molecules. Catalytic sensors transform the analyte to a different compound, yielding a detectable change of heat. A major disadvantage of catalytic sensors lie in their inability to detect compounds that are chemically stable.
Low density molecular sieves have been shown to be useful as selective chemical sensors. Zeolites are one of the several groups that comprise the structural family of microporous metal oxides known as molecular sieves. Zeolites are crystalline aluminum silicates that have three dimensional networks of connected channels or cages. A molecular sieve based sensor might be used to selectively detect those molecules which can readily absorb into the zeolite cages.
Zeolite molecular sieves have been employed as the active component in surface acoustic wave devices and piezo electric quartz crystal microgravometric type sensors. These approaches involve either a thin film configuration as a ceramic composite or the zeolites are tethered to an electrode surface via organic linkages. The sensing ability of these devices depends on changes in either mass or electrical properties produced by the absorption of the analyte. The sensing characteristics are limited in that a detectable mass change is required and the circuitry needed to detect a change in mass is relatively complicated. Further, these approaches cannot discriminate between analyte molecules of similar size and shape. Another problem is that it is very difficult to deposit uniform thin films of low density molecular sieve material in order to fabricate a reliable gaseous sensor. Therefore, it is desirable to have a gaseous sensor that has a uniform film layer and can discriminate between different types of absorbed analytes.